Here's what you need to know:
* Torrents are a file-sharing protocol. Basically, users download and "seed" torrents- once you've downloaded part of a file, you can re-upload that data to other people who need it, so everyone involved uploads what they have, and downloads what they need. So it has the advantage that it keeps working provided at least someone is still uploading, not necessarily whoever started it. It also has the advantage that you can pause and restart a torrent at will, something that doesn't work on most other protocols (for instance, if you don't finish downloading a torrent one day, you can turn your computer off, start up again the next day, and the torrent will keep going from where it left off).
* The more people involved, the faster it works. "Seeders" are people who are currently uploading- the more of them, the better. "Leechers" are people who basically download it, then drop out of the torrent to save bandwidth. Most people do this, as they don't want to waste bandwidth uploading later, although the torrent community frowns upon it.
* As for legality (and this is important)- there is nothing illegal about the torrent protocol itself; you can operate torrents without breaking any laws. HOWEVER, downloading copyrighted or other illegal material IS illegal. Most free movies are copyrighted, so those torrents are illegal to download. While a lot of people get away with it, you can be sued for it.
* You can also be sued if you operate your own torrent site where people share copyrighted material. While yes, it's technically legal to use torrents themselves, if your website is obviously being used to share copyrighted material, and you keep operating it, you can be busted for it. So I'd think twice before operating such a site.